We are having a huge table Table1(2.5 billion rows) with single column A(NVARCHAR(255) datatype). What is the right approach for seek operations against this table. Clustered index on A Vs Clustered Column store index on A.
We are already keeping this table in separate filegroup from the other table Table2, with which it will be Joined.
Do you suggest partitioning this table for better performance ? This column will have unicode data also. So, what kind of partitioning approach is fine for unicode datatype ?
UPDATE: To clarify further, the use case for the table is SEEK. The table is storing identifiers for individuals. The major concerns here are performance for SEEK in the case of huge table. This table will be referred inside a transaction. We want the transaction to be short.
Clustered index vs column store index depends on the use case for the table. Column store keeps track of unique entries in the column and the rows where those entries are stored. This makes it very useful for data warehousing tasks such as aggregates against the indexed columns, however not as optimal for transactional tasks that need to pull a small number of specific rows. If you are using SQL Server 2014 or later you can use both a clustered index and a columnstore index by creating a clustered columnstore index. It does have some limitations and overhead that you should read up on though.
Given that this is a seek for specific rows and not an aggregation of the column, I would recommend a clustered index instead of a column store index.
I just created a table with TWO primary keys in SQL Server. One column is age, another is ID number and I set the option to CLUSTER INDEX, so it automatically creates a cluster index on both columns. However, when I query the table, the results only seem to sort the ID and completely disregard/ignore the AGE (other PK and other Cluster index column). Why is this? Why is it only sorting based on the first cluster index column?
The query optimizer may decide to use the physical ordering of the rows in the table if there is no advantage in ordering any other way. So, when you select from the table using a simple query, it may be ordered this way. It is very easy to assume that the rows are physically stored in the order specified within the definition of your clustered index. But this turns out to be a false assumption.
Please view the following article for more details: Clustered Index do “NOT” guarantee Physically Ordering or Sorting of Rows
May I know how the include clause improves performance in a covering index?
CREATE NONCLUSTERED INDEX includeIndex
ON mytable(COL1)
INCLUDE(COL2,COL3,COL3)
and what is the difference between
CREATE NONCLUSTERED INDEX includeIndex ON mytable(COL1) INCLUDE(COL2,COL3,COL3)
and
CREATE NONCLUSTERED INDEX nonincludeIndex ON mytable(COL1,COL2,COL3,COL3)
Thanks
You can extend the functionality of nonclustered indexes by adding nonkey columns to the leaf level of the nonclustered index. By including nonkey columns, you can create nonclustered indexes that cover more queries. This is because the nonkey columns have the following benefits:
They can be data types not allowed as index key columns.
They are not considered by the Database Engine when calculating the number of index key columns or index key size.
An index with included nonkey columns can significantly improve query performance when all columns in the query are included in the index either as key or nonkey columns. Performance gains are achieved because the query optimizer can locate all the column values within the index; table or clustered index data is not accessed resulting in fewer disk I/O operations.
http://msdn.microsoft.com/en-us/library/ms190806.aspx
If your query does only query fields in the index (includes included fields) then after finding which rows to return the sql processor fdoes not have to load the actual data page to get the data. That simple. It can answer the query from the index only.
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What are the differences between a clustered and a non-clustered index?
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I need to add proper index to my tables and need some help.
I'm confused and need to clarify a few points:
Should I use index for non-int columns? Why/why not
I've read a lot about clustered and non-clustered index yet I still can't decide when to use one over the other. A good example would help me and a lot of other developers.
I know that I shouldn't use indexes for columns or tables that are often updated. What else should I be careful about and how can I know that it is all good before going to test phase?
A clustered index alters the way that the rows are stored. When you create a clustered index on a column (or a number of columns), SQL server sorts the table’s rows by that column(s). It is like a dictionary, where all words are sorted in alphabetical order in the entire book.
A non-clustered index, on the other hand, does not alter the way the rows are stored in the table. It creates a completely different object within the table that contains the column(s) selected for indexing and a pointer back to the table’s rows containing the data. It is like an index in the last pages of a book, where keywords are sorted and contain the page number to the material of the book for faster reference.
You really need to keep two issues apart:
1) the primary key is a logical construct - one of the candidate keys that uniquely and reliably identifies every row in your table. This can be anything, really - an INT, a GUID, a string - pick what makes most sense for your scenario.
2) the clustering key (the column or columns that define the "clustered index" on the table) - this is a physical storage-related thing, and here, a small, stable, ever-increasing data type is your best pick - INT or BIGINT as your default option.
By default, the primary key on a SQL Server table is also used as the clustering key - but that doesn't need to be that way!
One rule of thumb I would apply is this: any "regular" table (one that you use to store data in, that is a lookup table etc.) should have a clustering key. There's really no point not to have a clustering key. Actually, contrary to common believe, having a clustering key actually speeds up all the common operations - even inserts and deletes (since the table organization is different and usually better than with a heap - a table without a clustering key).
Kimberly Tripp, the Queen of Indexing has a great many excellent articles on the topic of why to have a clustering key, and what kind of columns to best use as your clustering key. Since you only get one per table, it's of utmost importance to pick the right clustering key - and not just any clustering key.
GUIDs as PRIMARY KEY and/or clustered key
The clustered index debate continues
Ever-increasing clustering key - the Clustered Index Debate..........again!
Disk space is cheap - that's not the point!
Marc
You should be using indexes to help SQL server performance. Usually that implies that columns that are used to find rows in a table are indexed.
Clustered indexes makes SQL server order the rows on disk according to the index order. This implies that if you access data in the order of a clustered index, then the data will be present on disk in the correct order. However if the column(s) that have a clustered index is frequently changed, then the row(s) will move around on disk, causing overhead - which generally is not a good idea.
Having many indexes is not good either. They cost to maintain. So start out with the obvious ones, and then profile to see which ones you miss and would benefit from. You do not need them from start, they can be added later on.
Most column datatypes can be used when indexing, but it is better to have small columns indexed than large. Also it is common to create indexes on groups of columns (e.g. country + city + street).
Also you will not notice performance issues until you have quite a bit of data in your tables. And another thing to think about is that SQL server needs statistics to do its query optimizations the right way, so make sure that you do generate that.
A comparison of a non-clustered index with a clustered index with an example
As an example of a non-clustered index, let’s say that we have a non-clustered index on the EmployeeID column. A non-clustered index will store both the value of the
EmployeeID
AND a pointer to the row in the Employee table where that value is actually stored. But a clustered index, on the other hand, will actually store the row data for a particular EmployeeID – so if you are running a query that looks for an EmployeeID of 15, the data from other columns in the table like
EmployeeName, EmployeeAddress, etc
. will all actually be stored in the leaf node of the clustered index itself.
This means that with a non-clustered index extra work is required to follow that pointer to the row in the table to retrieve any other desired values, as opposed to a clustered index which can just access the row directly since it is being stored in the same order as the clustered index itself. So, reading from a clustered index is generally faster than reading from a non-clustered index.
In general, use an index on a column that's going to be used (a lot) to search the table, such as a primary key (which by default has a clustered index). For example, if you have the query (in pseudocode)
SELECT * FROM FOO WHERE FOO.BAR = 2
You might want to put an index on FOO.BAR. A clustered index should be used on a column that will be used for sorting. A clustered index is used to sort the rows on disk, so you can only have one per table. For example if you have the query
SELECT * FROM FOO ORDER BY FOO.BAR ASCENDING
You might want to consider a clustered index on FOO.BAR.
Probably the most important consideration is how much time your queries are taking. If a query doesn't take much time or isn't used very often, it may not be worth adding indexes. As always, profile first, then optimize. SQL Server Studio can give you suggestions on where to optimize, and MSDN has some information1 that you might find useful
faster to read than non cluster as data is physically storted in index order
we can create only one per table.(cluster index)
quicker for insert and update operation than a cluster index.
we can create n number of non cluster index.
I am very beginner in SQL Server 2005 and I am learning it from online tutorial, here is some of my question:
1: What is the difference between Select * from XYZ and Select ALL * from XYZ.
2: The purpose of Clustered index is like to make the search easier by physically sorting the table [as far as I kknow :-)]. Let say if have primary column on a table than is it good to create a clustered index on the table? because we have already a column which is sorted.
3: Why we can create 1 Clustered Index + 249 Nonclustered Index = 250 Index on a table? I understand the requirement of 1 clustered index. But why 249?? Why not more than 249?
No difference SELECT ALL is the default as opposed to SELECT DISTINCT
Opinion varies. For performance reasons Clustered indexes should ideally be small, stable, unique, and monotonically increasing. Primary keys should also be stable and unique so there is an obvious fit there. However clustered indexes are well suited for range queries. Looking up individual records by PK can perform well if the PK is nonclustered so some authors suggest not "wasting" the clustered index on the PK.
In SQL Server 2008 you can create up to 999 NCIs on a table. I can't imagine ever doing so but I think the limit was raised as potentially with "filtered indexes" there might be a viable case for this many. Indexes add a cost to data modification operations though as the changes need to be propagated in multiple places so I would imagine it would only be largely read only (e.g. reporting) databases that ever achieve even double figures of non clustered non filtered indexes.
For 3:
Everytime when you insert/delete record in the table ALL indexes must be updated. If you will have too many indexes it takes too long time.
If your table have more then 5-6 indexes I think you need take the time and check yourself.